PURPOSE: To detect a spectral density distribution finely and to accurately display an image by providing plural photoelectric converting elements which are arranged outside an optical path of incidence on an image forming lens in parallel to scanning lines and detect the primary colors of light spectrally and a lens for a sensor having a focus on the photoelectric converting elements.

CONSTITUTION: The lens 90 for the sensor, shading 91, and photoelectric converting element 92 are provided nearby an optical unit 42, and color filters of C, M, and Y are arranged as one group on the surface of the photoelectric converting element 92 to form plural detection parts. The lens 90 for the sensor has focal length for forming image light of a document 32 in a main scanning direction on the photoelectric converting element 92 and spectral density can be measured through the color filters. Namely, a prescan is made before image forming operation, a spectral density measurement signal from the photoelectric converting element 92 is digitized and stored, and an arithmetic processing circuit controls an exposure quantity and the combination of filters in the optical unit 42 and makes secure color corrections to represent an accurate image.